Scintillation detectors are well known and are used to detect ionizing radiation, such as gamma or x-rays produced from a source. Applications include detecting the density and level of materials in containers, as well as identifying elements based on emitted spectra or forming images of items through which the ionizing radiation passes (for example, medical or industrial imaging). Such scintillation detectors include a scintillator, which emits light in response to receiving a quanta of the ionizing radiation, and a light detector which generates an electrical signal in response to light from the scintillator. The scintillators may be a transparent crystal, such as a NaI or CsI crystal, typically doped (for example with thallium), though many other materials are known (for example, bismuth germanante, gadolinium silicate, and others). While various light detectors are also known the PMT (PMT) is commonly used in view of its relatively low cost, ease of operation, and sensitivity. The electrical pulses produced by PMTs may be counted by, for example an electronic counter. The density of at a location of an item can be estimated by counting pulses resulting from ionizing radiation which has passed through or been scattered from the item. Also, in other applications these output pulses can be analyzed and a pulse distribution at different energies may be obtained (an “energy spectrum”). Typically this will be a spectrum of pulse counts at each energy (or amplitude). Distinct peaks at each energy level can be evaluated and elements identified based on the energy spectrum. Applications of the foregoing types are well known.
The PMT is a highly sensitive detecting device for converting light into amplified electrical signals. A typical PMT includes an evacuated glass tube and a series of electrodes disposed within the tube. The series of electrodes includes a photocathode from which a light source enters the tube, a focusing electrode, a plurality of dynodes that function as an electron multiplier, and an anode where the multiplied charge accumulates. In operation, a high voltage source is used to hold each successive dynode at a higher voltage than the previous dynode, with the anode being at the highest potential. When incident photons (incident light) strike the photocathode of the PMT, the photons eject photoelectrons due to the photoelectric effect. The photoelectrons emitted from the photocathode are accelerated by an electric field, and are directed toward the electron multiplier (the series of dynodes) by the focusing electrode. The electron multiplier multiplies the photoelectrons by a process of secondary emission. When the multiplied photoelectrons reach the anode, they are output as an electrical signal.
More specifically, when the accelerated photoelectrons strike the first dynode, secondary electrons are emitted through secondary emission. These secondary electrons join the first batch of photoelectrons and are accelerated toward the next dynode. This process is repeated over successive dynodes. This cascade effect of secondary emission results in an increasing number of electrons produced at each successive dynode. In other words, charge is amplified at each successive dynode. When the electrons reach the anode, they are output as an amplified electrical signal. As a result of the above process, even a small photoelectric current from the photocathode can provide a large output current at the anode of the PMT. The amplification, which may be referenced as “gain” depends on the number of dynodes, accelerating voltage, temperature, and the like.
PMTs provide advantages such as high internal gain, high sensitivity, fast responses, low noise, and a high frequency response. However, the gain of scintillators and light detectors such as the PMT may fluctuate due to various factors such as temperature and age. This can lead to variable PMT output over time even from the same received ionizing radiation, which in turn can lead to misinterpretation of results.